Push-pull fluorescent dimming circuit

Abstract

A diming circuit for a hot cathode fluorescent lamp is operated in a push-pull mode whereby the hot filaments are alternately the cathode and the anode. Both filaments thus experience similar degradation with time at all brightness levels, thereby enhancing and lengthening the life of the lamp.

What is claimed as new and desired to be secured by Letters Patent of the United States is: 
     
       1. Variable arc-voltage power supply circuitry for a fluorescent lamp light source comprising: (a) fluorescent lamp means having a plurality of filament electrodes,   (b) a filament current supply source coupled to and heating said lamp filaments,   (c) a push-pull DC arc voltage supply for alternately producing arc supply voltage pulses of opposite polarity including switch means coupled between a DC supply and the primary winding of a transformer the secondary winding of which is coupled to said electrodes; said switch means being opened and closed to permit current flow in one direction to generate a voltage pulse of one polarity in said secondary winding and alternately opened and closed to permit current flow in the opposite direction to generate a voltage pulse of the opposite polarity,   (d) means for coupling said pulses to said lamp electrodes whereby each of said filament electrodes alternately operates as an anode and as a cathode, whereby both filament electrodes are subject to even wear,   (e) dimming means for said lamp means including means for varying the duty cycle of the output pulses from said arc voltage supply to vary lamp brightness.   
     
     
       2. The power supply according to claim 1 wherein said arc voltage supply includes a switched fly-back transformer means for generating the arc voltage output pulses and means for varying the switching rate to vary the duty cycle of said arc voltage output pulses. 
     
     
       3. The power supply according to claim 2 including a D.C. voltage supply and switch means coupled to the primary winding means of said fly-back transformer for alternately connecting and disconnecting said D.C. supply from said primary winding means thereby selectively initiating current flow in opposite direction through said winding means. and interrupting current flow to produce arc voltage output pulses of opposite polarity. 
     
     
       4. A power supply according to claim 3 wherein said primary winding means has two series connected winding sections with said D.C. voltage supply coupled to the junction of said sections, individual switch means connected to said winding sections for alternately initiating and terminating current flow through said winding sections to produce said output pulses of opposite polarity, and control pulse means coupled to said individual switch means for alternately closing and opening said switch means to initiate opening current flow upon closure of an individual switch means and to terminate current flow upon opening of said individual switch means. 
     
     
       5. The power supply according to claim 4 including means to vary the switching control pulse rate to vary the switching rate and the duty cycle of said arc voltage output pulses.